The present invention relates to an optical system installed in a color copier of the type having a plurality of photoconductive elements, particularly at lest three photoconductive elements, in order to split its optical path.
In the art of color copiers, two different types of optics are known: one in which two half-mirrors and a single total reflection mirror are positioned in an optical path, along which an imagewise reflection from a document is propagated, in an angular position of 45 degrees each so as to split the optical path into three, the amounts of light of different colors being balanced by the half-mirrors, color separating filters, and neutral densitys (ND) filter; and one in which colored half-mirrors are used to effect the division of an optical path and color separation at the same time. A problem with any of such prior art systems is that the two half-mirrors each being inclined 45 degrees critically deteriorate the image-forming ability of a lens and, therefore, the quality of image reproduction even to an optically unallowable level. Another problem is that the ND filters are indispensable for the total sensitivity to the different separated colors of light to be balanced.
Further, when the incident angle to such half-mirrors which are installed in the optical system of a color copier is large, the half-mirrors are apt to reflect even needless color components, resulting in the deterioration of color separating ability. In addition, secondary reflections from the back of the half-mirrors are unwantedly introduced into the reflection path to bring about double exposure.
Another implementation heretofore proposed to split the optical path into three segments is the use of a dichroic mirror in place of the half-mirrors. In case that a dichroic mirror which does not use transmitted light, i.e., a light intercepting mirror is used, a shutter or like light intercepting means has to be provided behind the mirror. Such an implementation, however, duffers from a drawback that the incident light is reflected not only by the front surface of the mirror (primary reflection) but also by the back of the mirror (secondary reflection), the secondary reflection introducing noise in the primary reflection to cause double exposure and lower color separating ability. While the secondary reflection may be excluded by providing a non-reflective coating on the back of the mirror, the coating often amounts to several tens of layers for functional reasons and, yet, providing such a non-reflective laminate coating requires disproportionate cost. Color separating filters extensively used to separate blue, green and red from three split beams of light include Wratten Nos. 47, 58 and 25 available from Kodak, and BPB-45, BPB-53 and SC-60 available from Fuji Photofilm. Such filters are generally classified into two types, i.e., a filter constituted by a triacetate film which is provided with a color characteristic, and a filter constituted by a transparent glass on which a laminate coating is provided. The film type filter is inexpensive, but its transmission is low and light amount loss is great. The glass type filter, on the other hand, feature inherently high transmission, but it cannot cut the components appearing at both sides of a necessary wavelength range, unless the laminate coating is made up of a twice greater number of layers than in the case where the wavelengths at only one side is to be cut, at the sacrifice of cost.
It has been customary to selectively use ND filters, which serve to balance the total sensitivity to blue, green and red separated colors, which have certain transmission. When the voltage applied to a lamp of an illuminating unit is changed as in the case of a reduce copy mode or an enlarge copy mode operation, the transmission of the ND filter has to be changed to preserve the balance between the three different colors. However, the transmission of each ND filter is generally fixed and difficult to change it to another desired value.
Meanwhile, assume that a photoconductive element which is exposed imagewise through a first half-mirror positioned in the above optical path is a first photoconductive element, a one which is done so through a second half-mirror is a second photoconductive element, and a one which is done so through a third half-mirror is a third photoconductive element. Assuming that the first and second half-mirror are 3 millimeters each, the magnification errors and optical resolutions, or MTF (Modulation Transfer Function), of images which are projected onto the respective photoconductive elements are as shown below in Table 1.
TABLE 1 __________________________________________________________________________ ITEM UNDER CONSTANT UNDER OMTIMUM MAGNIFICATION MTF CONDITION CONDITION FOCUSING MAGNIFICATION MAGNIFICATION POSITION ERROR MTF ERROR MTF __________________________________________________________________________ 1ST ELEMENT -0.14% 52% 0 41% 2ND ELEMENT -0.28% 45% 0 24% 3RD ELEMENT 0 32% 0 32% __________________________________________________________________________
It will be seen that under the optimum resolution condition the second photoconductive element suffers from a magnification error of -0.28% which means a contraction of image by 0.42 millimeters, compared to an image formed on the third photoconductive element. In a color copier, since toner images of different colors are superposed on a paper to reproduce a color image, any difference in magnification between the colors translates into misalignment of the images of respective colors, resulting in poor image quality. Furthermore, since the first and second half-mirrors are disposed in the optical path which terminates at the third photoconductive element, should the images projected onto the first to third photoconductive elements be of the same magnification, the MTF which are required to be greater than 50% would be lowered beyond 50% to result in a blurred image, as seen from Table 1.